The present invention evolved from continuing development efforts pertaining to subject matter disclosed in my co-pending application Ser. No. 265,128 filed May 19, 1981, entitled "Symmetric Microwave Mixer". This co-pending application discloses a diode mixer with compact, symmetric conductor port structure significantly reducing unwanted sensitivity to adverse parameters.
The present invention relates to improvements in the microstrip circuit layout and structure for those applications where even greater isolation is desired. The mixer port structure of the present invention enables implementation of a four diode mixer with first and second separate, isolated sections. Each section comprises a pair of diodes interconnecting a respective half of a dual balun and a respective conductor.
In a particularly desirable aspect of the invention, mixer section separation and isolation is achieved without sacrificing compactness or symmetry of the mixer port structure. The mixer port structure of the present invention significantly reduces unwanted sensitivity to adverse parameters, and also overcomes problems of distortion caused by unequal conduction paths and diode lead lengths. Transmission lengths along the microwave conductors leading to the diode mixer sections are minimized to reduce loss and exposure to mismatch influences. Transmission path lengths through the diodes are also minimized, and are substantially equal to thus eliminate unwanted phase shift due to differential path lengths through the mixer diodes. The diode mixer leads are short and orthogonal, to provide good impedance matching and minimize unwanted coupling.
The microstrip circuit layout and structure further enables coplanar diode connection. This eliminates the need to extend diode leads through the substrate, and is desirable in applications where reduction of assembly costs has high priority.
The operating bandwidth of the mixer is significantly increased by the separate, isolated mixer sections, the balanced port structure, the equalized conduction path lengths, and the short lead lengths.
In another desirable aspect of the invention, standard printed circuit board techniques may be used for fabrication, resulting in cost effective manufacture. The invention also enables the use of low dielectric constant substrate material, further reducing cost.